If the frame is steel the clearcoat will have a hard time bonding to the bare steel. I did this with the frame I built and after a couple months it started to flake and chip. Surface rust started to form where it chipped and spiderwebs of rust started to show up. It does look cool but is not a long lasting option. Another thing I tried was using Gun Blue metal dye. It makes the steel a very dark blue color, while the silver and brass used for brazing comes through looking pretty much the same. Again it looks cool, but it is a lot of work to keep it from rusting. Once it gets wet you need to clean it, and oil it with a good penetrating oil. Even clear powdercoating has trouble on bare steel. After a year or so, spiderwebs of surface rust start to form. Your best bet would be a decent wet paint, or a good powdercoating job. If the frame is stainless steel you can polish it up, but it is also a bit of a hassle to keep looking good(lots of polishing).

Ti can be left bare with no problems, so can aluminum. You can also polish the aluminum.

theres a guy around here who stripped all the paint off his iro and used some spray clear coat over the steel and thats worked for the past couple of winters.. but maybe he incorporated some kind of voodoo magic penetrating oil

One thing I have not tried is Gibbs Brand Penetrating Oil. Supposedly it works well on undercarriage parts on cars. Also it says you can paint over it with no problems. Maybe that would be a good option.